1. Field of the Invention
The present invention relates to forming a metal silicide film, and more particularly, to a method of depositing a metal silicide film that is robust to stress cracks in subsequent thermal processing.
2. Description of the Related Art
Refractory metal silicides are attractive materials for semiconductor fabrication because of their high melting points and low resistivities. In particular, these materials are popular for use in the formation of integrated circuits as intermediate barriers or conducting films, runners or wiring. Metal silicides are also used for a polycide gate structure composed of a polysilicon layer and a metal silicide layer used for the gate electrode.
Metal silicides are typically formed by a deposition process such as chemical vapor deposition (CVD) or sputtering. For example, a forming tungsten silicide by a conventional CVD process involves the reaction between silane (SiH.sub.4) and tungsten hexafluoride (WF.sub.6). In order to avoid gas phase nucleation of the metal silicide on the walls of the reaction chamber, the ratio of SiH.sub.4 to WF.sub.6 is kept high, on the order of about 50:1. Tungsten silicide formed by this reaction is non-stoichiometric WSi.sub.x, with the "x" factor depending on the silane flows and generally having a value of between about 2 and 3.
One problem with the deposition of metal silicides is that the resulting film often has high stress. For example, a tungsten silicide film formed by the conventional CVD process described above will have high residual tensile stress. Because such films are deposited at elevated temperatures, upon subsequent cooling of the film, a tensile stress is created by the differences in thermal expansion coefficients between the deposited film and an underlying substrate. Furthermore, stresses also arise in the deposited material due to defects such as dislocations in the resulting crystal structure. Because of the residual stress of the material, underlying topography and thermal processing of the device after WSi.sub.x deposition makes WSi.sub.x interconnects susceptible to stress cracks and material failure. While it has been found that increasing silane flows can reduce this stress, increasing silane results in poor step coverage for the film.
Accordingly, what is needed is a metal silicide film for a semiconductor device having low internal stress. In particular, this metal silicide film should be robust to subsequent thermal processing to prevent the formation of stress cracks in the film that may lead to material failure.